In the past, evanescent wave sensors have generally been limited to fiber/waveguides, microspheres, and microdisks. The older fiber/waveguide sensors required large sample volumes. The relatively newer stand-alone microspheres and microdisks (or microrings) fabricated on a substrate were smaller and had greater sensitivity than the fiber/waveguides, but microspheres are difficult to mass-produce and micro-disks (or microrings) suffered from degraded Q-factors due to surface roughness created during the fabrication process. Importantly, all of the evanescent wave sensors suffered from problems associated with efficiently combining fluidics and photonics, such that the two aspects did not interfere with one another (e.g. issues relating to effective fluid delivery, optical signal reliability, and high-throughput capability). These problems were related to the fact that the evanescent field sensors of the past attempted to detect compositions located on the exterior of the sensor. None of the evanescent wave sensors in the prior art contemplated using an optical ring resonator with a hollow core, which has the capability of detecting compositions on the interior surface of the sensor.
The first generation of optical evanescent-wave sensors, fiber/waveguide sensors, has been in existence for over twenty years and has found applications in many fields [1]. These devices have been characterized as being immune to electromagnetic waves and capable of performing remote sensing. A generic configuration of a first generation fiber/waveguide sensor is depicted in FIG. 1. In the illustration, the outside surface of the fiber/waveguide was first immobilized with a layer of biorecognition molecules such as antibodies. The guided light traveling along the inside of the fiber/waveguide had an evanescent field extending outwardly from the sensor into the surrounding medium (e.g., water) for approximately 100 nm and was capable of interacting with the bio/chemical molecules near the fiber/waveguide external surface. The guided light changed in response to tiny modifications in refractive index near the fiber/waveguide surface when target analytes were captured; the modifications in the optical signal could be detected at the output as a sensor signal. The sensitivity was determined by light-analyte interaction since the sensing signal was accumulative in nature. A longer interaction length resulted in higher sensitivity, and hence a lower detection limit. In these fiber/waveguide sensors, the light passed the fiber/waveguide only once. Consequently, fiber/waveguide sensors were required to be a few centimeters in length so they could achieve the adequate sensitivity [2,3]. The length and bulky characteristics of fiber/waveguide sensors also created significant problems by increasing the required sample volume and reducing the sensor multiplexing capabilities.
One solution to the short interaction problem of fiber/waveguide sensors was to have the light at the output coupled back to the input in order to recycle the light. This idea evolved into “ring resonator sensors,” in which the light circulates at the inner surface of the ring-shaped waveguide repetitively. This circulation mode of the light was called the “whispering gallery mode” (WGM) [4]. In a ring resonator, whispering gallery modes (WGMs) form due to total internal reflection of the light along the curved boundary surface. In a ring resonator, the evanescent field of the WGM extends into the surrounding medium for approximately 100 nm and is capable of interacting with the molecules on the ring resonator surface in the same manner as in fiber/waveguide sensors.
Optical ring resonator sensors have been characterized by their remote sensing capabilities, improved sensing performance, and immunity to electromagnetic waves. As compared to a simple fiber/waveguide design, the resonating nature of the circulating light has significantly enhanced the light-analyte interaction, and hence the sensitivity. Furthermore, the effective light-analyte interaction length of a ring resonator has not been limited by the sensor physical size, but rather by the number of circulations of the light supported by the ring resonators, which is characterized by the resonator quality factor, or Q-factor parameter.
Generally, there have been two types of ring resonators used in sensor development—based on either microsphere shaped or microdisk/microring shaped structures (FIG. 3). Since microdisk and microring structures are very similar, for simplicity only the microdisk resonator is used as an example. The Q-factor ranges from 103 to 104 for microdisk based ring resonators and exceeds 106 for microsphere based ring resonators [4-13]. Although the size of those ring resonators is only a few tens to a few hundreds of microns in diameter, the effective interaction length can be 10 cm to 1,000 cm due to the high Q-factor. Thus, a ring resonator can deliver sensing performance equivalent to a waveguide while using orders of magnitude less surface area, which results in a significantly reduced sample volume. Furthermore, due to small size of the ring resonators, high-density sensor integration becomes possible.
Recently, the detection of bio/chemical molecules such as protein, DNA, enzyme, and mercuric ions with microsphere ring resonator sensors has been demonstrated by several research groups [6-13]. The detection limit of biomolecules and the refractive index change of solutions respectively are shown to be on the order of 1 pg/mm2 and 107 refractive index units (RIU), much better than that of fiber/waveguide sensors [2,3,9,11-13].
Ring resonator sensor development encompasses two essential parts, photonics and fluidics, which deal with delivering the light and the aqueous samples to the sensing head, respectively. It is important that the fluidics portion is incorporated without sacrificing the photonic sensing performance such as multiplexing capability and high Q-factor. While disk-shaped ring resonators can be mass-produced with photolithographic technologies in an array format, they suffer from a degraded Q-factor due to surface roughness created during the fabrication process. Moreover, an effective fluidics system that is separately fabricated from the photonics has yet to be demonstrated with microdisks.
On the other hand, microsphere resonators have much higher Q-factors. Nevertheless, integration of microsphere arrays and the subsequent incorporation of fluidics, however, have proven to be very challenging because of the spheres' 3-D configuration and fabrication process. As a result, it appears that neither of these ring resonator technologies is optimal for practical sensing systems.
Therefore, there is a need for novel sensor architecture to incorporate the fluidics without sacrificing the photonic sensing performance. There is also a need for a better method of production, which does not suffer from surface roughness and degraded Q-factor. There is also a need for novel sensor architecture for easy and practical fabrication. There is yet another need for novel sensor architecture capable of densely multiplexing into a small array to allow for simultaneous detection of multiple target analytes, as well as redundant testing. The present invention has unique structural components as well as functional attributes that solve the problems associated with combining photonics and fluidics. Moreover, the present invention does not suffer from degraded Q-factor, which has been a recognized problem in developing ring resonators, typically due to surface roughness induced during the fabrication processes. Accordingly, there is a significant need for the hollow core optical ring resonator of the present invention, which provides a structure that is capable of incorporating fluidics without sacrificing photonic sensing performance.